I need to create a bootable Mac OS installation of High Sierra on an external drive to run an application that doesn't work on Mojave, which is what is installed on my Macbook Pro. I can't find High Sierra Installation application on the App Store. Where do I get it?
MacBook Pro 15", macOS 10.14
Try here, How to upgrade to macOS High Sierra - Apple Support scroll down to Section 4 and click on Get macOS.
If you are running Mojave you might be faced with a refusal, so download it on another Mac with a lower OS, but use the Apple ID for the Mac you want to use High Sierra on.
If needed, you can make a bootable USB stick to install using these free programs which will do all the work for you.
Bootable USB Flash Drive – Diskmaker X Disk Creator or use Apple’s method Create a bootable installer for OS
MacOS High Sierra installation application
